The present invention relates to an ink composition for electrostatic attraction (suction) ink jet and to an electrostatic attraction ink jet recording process using the ink composition.
Ink jet recording process allows high speed printing with low noise and has recently spread rapidly.
An ink jet printer is adapted to jet a liquid ink having a high fluidity through a fine nozzle onto a recording paper to perform recording. The ink jet printer operates either in an on-demand (voluntary jetting) process or continuous jetting process. As such continuous jetting processes there are known electrostatic jetting processes (e.g., Sweet type, Hertz type). As such on-demand jetting processes there are known piezoelectric process, thermal ink jet process and electrostatic acceleration process.
As the on-demand ink jetting processes using electrostatic force there is known a process so-called electrostatic acceleration ink jet process or slit jet process as disclosed in Susumu Ichinose and Yuji Oba, xe2x80x9cBulletin of Society of Electronic Communications of Japanxe2x80x9d, vol. J66-C (No. 1), page 47, 1983, Tadayoshi Ono, Ei Mizuguchi, xe2x80x9cBulletin of Society of Image Electronics of Japanxe2x80x9d, vol. 10, (No. 3), page 157, 1981, etc. Specific examples of these type processes are disclosed in JP-A-56-170 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d), JP-A-56-4467, and JP-A-57-151374.
In this type of ink jet printer, an ink is supplied from an ink tank into a slit ink nozzle or the like comprising a slit ink retaining portion having many electrodes disposed inside thereof, while a high voltage is selectively applied to these electrodes, to thereby jet the ink in the vicinity of the electrodes onto a recording paper disposed closely opposed to the slit.
As one of other types of ink jet recording processes free of slit recording head there is disclosed in JP-A-61-211048 a process which comprises filling pores in a film-like ink support having a plurality of micropores with an ink while a voltage is selectively applied from multi-needle electrodes, to thereby transfer the ink in the pores to a recording paper.
Explaining the mechanism of ink flying, it is interpreted that the high voltage applied to the electrodes disposed causes electric charge to be injected into the ink in contact with these electrodes to thereby electrostatically charge the ink in the vicinity of these electrodes, and hence an electrostatic force is caused to jet the ink. Accordingly, the ink is normally not electrostatically charged. Only when a voltage is applied to these electrodes, the ink is electrostatically charged in the vicinity of these electrodes by energizing, thus obtaining a jetting force. The ink for use in these ink jet recording processes has an electrical resistance of from about 105 to 106 xcexa9xc2x7cm. Since water has a low electrical resistance, an ink obtained by dispersing a coloring agent such as dye in an oily solvent with a dispersing aid such as surface active agent for adjusting the electrical resistance thereof is generally used.
Referring to the oily ink for use in this type of ink jet recording process, there have been proposed a process which comprises controlling the viscosity and specific resistivity of the oily ink (as disclosed in JP-B-52-13127 (The term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent publicationxe2x80x9d)), a process which comprises controlling the dielectric constant of the dispersion medium to be incorporated in the ink and the specific resistivity of the ink (as disclosed in JP-A-53-29808), a process which comprises changing the kind of the dispersion medium to be incorporated in the oily ink or allowing a specific compound to be incorporated in the oily ink as an ink composition (as disclosed in JP-A-3-79677, JP-A-3-64377, JP-A-4-202386, JP-A-7-109431), etc.
However, the conventional techniques leave something to be desired. For example, the oily ink used in these conventional techniques leaves something to be desired storage stability, reproducibility of recorded image upon repeated use, resistance to ink feathering on transfer material, anti-clogging property in nozzle and ink supply passage, stability in ink jetting, etc. Further improvement in these properties has been desired.
On the other hand, another electrostatic suction ink jet technique is disclosed in WO93/11866. This process involves the supply of an ink obtained by dispersing, in an insulating liquid, charged particles or particles which are rendered electrostatically chargeable under the application of electric field into the ink jetting apparatus. Under the application of electric field, a continuous process is effected comprising the steps of (i) forming an ink meniscus at the forward end of a jetting electrode for jetting ink, (ii) allowing the electrostatically chargeable particles in the ink meniscus to be electrophoretically migrated to cause agglomeration (concentration), and (iii) forming an electric field between a counter electrode on which a recording medium is placed and the jetting electrode, to thereby jet the electrostatically chargeable particles thus agglomerated.
Unlike the conventional types of ink jet recording process, this type of ink jet recording process requires no ink nozzle structure. Thus, this type of ink jet recording process can jet an ink comprising dispersed particles of pigment, etc. in the form of fine droplet having a size of about several micrometers. Further, a high concentration droplet as the result of the agglomeration of dispersed particles can be jetted. Moreover, by controlling the jet signal, the size of droplet can be changed, making it possible to change the size of dots in the image.
Accordingly, in accordance with this type of ink jet recording process, an image based on light-resistant and water-resistant pigments and an image comprising a continuous dot gradation can be sharply formed with a high resolution and a high density.
As the oily ink to be used in this type of ink jet recording process WO95/1404 and WO96/10055 disclose an oily ink comprising insoluble and electrostatically chargeable particles and a charging agent each incorporated in an insulating liquid having an electrical resistance of not less than 109 xcexa9xc2x7cm.
Further, an oily ink comprising electrostatically chargeable particles having a specified chargeability or specified average particle diameter has been proposed (as disclosed in JP-A-9-193389, JP-A-8-291267). Moreover, an oily ink which exhibits specified thermal properties when dried has been proposed (as disclosed in JP-A-9-137094).
However, these proposed oily inks are disadvantageous in that they leave something to be desired in jettability in ink jet recording. For example, they cause portions where the oily inks cannot be jetted upon application of a voltage corresponding to the jet signal. Further, these oily inks are occasionally jetted with its dispersed particles being insufficiently agglomerated, causing feathering or lack in density on dots printed on the transfer material. Moreover, in case of changing the application time of pulse voltage during jetting to change the size of dots thus printed to thereby form a continuous gradation, these proposed oily inks cannot be jetted at an efficiency of 100% when the pulse time is reduced to print small dots, making it impossible to form a high resolution image.
On the other hand, with the recent development of office machines and office automation, an offset lithographic printing process has spread in the art of light printing which comprises plate-making or forming an image by various methods on a lithographic printing plate precursor comprising an image-receiving layer provided on a water-resistant support to prepare a printing plate. As one of these plate-making processes there is also practiced an ink jet recording process.
When the foregoing oily ink is printed on such a lithographic printing plate precursor using the foregoing electrostatic suction ink jet recording process, several hundreds of sheets having a sharp image free of lack can be obtained at largest, demonstrating that these proposed oily inks leave something to be desired. In other words, these oily inks are disadvantageous in that the ink particles show an insufficient fixing strength when used in offset printing.
It is therefore an object of the present invention to provide an ink composition for electrostatic attraction ink jet recording, which provides excellent ink jet stability and provides a fair and high precision fine sharp image having excellent image strength.
Another object of the present invention is to provide an ink composition for electrostatic attraction ink jet recording which enables printing of a large number of sheets of printed matters having a sharp image.
A still other object of the present invention is to provide an improved electrostatic attraction ink jet recording using the ink composition.
Other objects and effects of the present invention will become apparent from the following description.
The above-described objectives of the present invention have been achieved by providing the following ink compositions and ink jet recording process.
(1) An ink composition for electrostatic attraction ink jet, which composition comprises:
a nonaqueous solvent having an electrical resistance of not lower than 109 xcexa9 and a dielectric constant of not more than 3.5;
a dispersoid dispersed in said nonaqueous solvent, which comprises at least a solid particulate material having an average particle diameter of from 0.1 to 3 xcexcm; and
at least one fluorine-based surface active agent soluble in said nonaqueous solvent, said surface active agent being present in an amount of from 0.05 to 5% by weight based on the weight of the ink composition.
(2) The ink composition according to the above (1), wherein said solid particulate material is a positively or negatively charge detectable particulate material.
(3) The ink composition according to the above (1) or (2), wherein said solid particulate material comprises a particulate resin material which is hydrophobic and which stays solid at least at ordinary temperature.
(4) The ink composition according to any one of the above (1) to (3), wherein said fluorine-based surface active agent is a polymer compound comprising a polymer component containing a fluorine-containing substituent having a weight-average molecular weight of from 800 to 30,000.
(5) An electrostatic attraction ink jet recording process, which comprises the steps of:
supplying an oily ink composition into a jetting portion; and
applying an electrostatic field to said jetting portion to jet said oily ink composition,
wherein said ink composition is an ink composition according to any one of the above (1) to (4).